Coordinate measuring machines are used for dimensional inspection of workpieces such as machine parts. A workpiece is secured to a fixed table, and a measuring probe is secured to a vertically movable ram which is also movable in a horizontal plane. To measure the position of a point on the workpiece, the probe is brought into contact with the point and the x, y and z measuring scales of the machine are read. To measure a distance between two points, the points are contacted successively, the coordinates of both points are read and distance is calculated from the coordinates. State of-the art coordinate measuring machines have refinements such as high resolution measuring systems, electrical contact probes, motor drives, computer controlled drives and computer acquisition and processing of data.
The accuracy of a coordinate measuring machine is limited by inaccuracies in the scales or other measuring systems and by faults in the guideways which establish orthogonality of machine motions. One approach to increasing accuracy is simply to improve the construction techniques and reduce tolerances of the system so that errors are reduced. However, the reduction of errors becomes progressively more expensive as required accuracies increase. Another approach is direct measurement of x, y and z errors at points throughout the machine working volume. This approach is impractical because of the huge amounts of data which must be stored for large machines and because of the time required to measure such data. A third approach is the measurements of errors in parametric form. That is, sets of error parameters are measured, for example, along three mutually orthogonal axes and stored for future use. The x, y and z errors at any point in the measurement volume are calculated from the parametric errors. The calculated errors are then subtracted from the scale readings to determine actual workpiece coordinates.
The coordinate measuring machine has three sets of guideways which establish probe motion. Ideally, movement along each of these guideways should result only in linear motion and the scale reading would equal the linear displacement. In reality, however, there are scale errors and the guideways are not completely straight or perfectly free from twist. For a real machine, there are six degrees of freedom which produce errors during movement along each guideway. For each direction of movement, there are three linear errors, Dx, Dy and Dz and three rotational errors, Dx, Dy and Dz and three rotational errors, Ax, Ay and Az. These six error parameters can be measured at a number of points along each direction of machine movement, resulting in an error matrix with 18 error parameters. From the matrix of 18 error parameters, the error at any point in the measurement volume can be calculated.
Various techniques have been used for the measurement of parametric errors. Laser interferometer techniques are well known for measuring displacement errors with high accuracy. Dual frequency interferometer techniques have also been utilized for measurement of straightness and roll as disclosed in U.S. Pat. No. 3,790,284, issued Feb. 5, 1974 to Baldwin. A system utilizing partitioned photocells to detect pitch and yaw of a stage is disclosed in U.S. Pat. No. 3,715,599 issued Feb. 6, 1973 to Marcy. A four quadrant angular movement sensor is disclosed in U.S. Pat. No. 3,765,772 issued Oct. 16, 1973 to Willett. One prior art approach to measurement of parametric errors utilizes a Hewlett-Packard 5526A laser measuring system, which is described in "Calibration of a Machine Tool," Hewlett-Packard Laser Measurement System Application Note 156-4. The system is transportable between machines but the machine calibration time is about 40 hours. In addition, a different setup is needed for each measurement and setup errors are difficult to avoid. A system for measuring the six error parameters along each axis of motion of a measuring machine is disclosed in U.S. Pat. No. 4,261,107, issued Apr. 14, 1981 to Coleman et al. The system utilizes interferometric techniques for measuring each of the error parameters and requires a dual frequency laser to measure displacements perpendicular to the laser beam axis. As a result, the system is complex and expensive. Furthermore, different fixed measurement arrangements are utilized for each of the three axes of motion of the machine, thereby further adding to the complexity and cost of the system.
In the measurement of error parameters for a coordinate measuring machine, the machine elements are normally assumed to be rigid bodies that do not undergo deflections, or deformations, as the probe is moved. A careful analysis, however, reveals that certain elements of the machine deflect as a function of probe position.
A bridge coordinate measuring machine includes y-guideways for support of the bridge. The y-guideways are typically supported at each end, and thus are deflected downwardly by the weight of the bridge. The bridge carries the x-carriage and the z-ram, with the probe attached to its lower end. The deflection of each y-guideway depends both on the position of the bridge and the position of the x-carriage on the bridge. When the x-carriage is closer to one end, the y-guideway on that end is deflected more.
Nonrigid behavior is also exhibited by other types of coordinate measuring machines. In a horizontal arm machine, a horizontal ram is supported by a z-carriage on a z-rail. The z-rail is deflected from vertical depending on both the position of the z-carriage and the distance by which the arm is extended.
It is a general object of the present invention to provide improved methods for calibrating coordinate measuring machines.
It is another object of the present invention to provide methods for calibrating coordinate measuring machines that compensate for nonrigid behavior of machine elements.
It is a further object of the present invention to provide methods for calibrating the position of a movable element relative to a fixed element in a machine which is deformable.
It is another object of the present invention to provide methods for coordinate measuring machine calibration wherein nonrigid error parameters are determined from error measurements along parallel, spaced-apart measurment lines.
It is yet another object of the present invention to provide methods for calibrating coordinate measuring machines with high accuracy.